1. Field of the Invention
The present invention relates to a liquid crystal projector apparatus for projecting an image using a liquid crystal panel, and more particularly, to a structure and method for cooling the liquid crystal panel and polarizing plate.
2. Description of the Related Art
Liquid crystal projector apparatuses have been dramatically improved in image quality, resulting from an increase in the luminance of a light source, refinement of liquid crystal light valves, and the like, and are now utilized widely from home theater applications to business presentations.
FIG. 1 illustrates the basic configuration of a liquid crystal projector apparatus. Liquid crystal projector apparatus 1 comprises three optical systems including illumination optical system 2, color separating optical system 7, and light composing optical system 13.
Illumination optical system 2 comprises light source 3 including a high luminance lamp such as a metal halide lamp, an ultra-high pressure mercury lamp, or the like; reflector 4 for reflecting light from light source 3; optical integrators 5a, 5b each for uniformarizing a luminance distribution of reflected light from reflector 4; and polarization beam splitter (PBS) 6 for transforming randomly polarized light from light source 3 into linearly polarized light.
Color separating optical system 7 comprises dichroic mirrors 8a, 8b each for separating an entire light flux from illumination optical system 2 into individual color light fluxes of red (R), green (G), and blue (B) and directing the separated color light fluxes to respective liquid crystal panels corresponding thereto; reflective mirrors 9a, 9b, 9c; and relay lenses 10a, 10b. 
Light composing optical system 13 comprises light modulator 14 for modulating the respective color light fluxes applied from color separating optical system 7 in accordance with given image information; color combiner prism 15 for combining the modulated color light fluxes; and projection lens 16 for projecting the combined light flux onto a screen.
Among the foregoing components, light modulator 14 in light composing optical system 13 comprises liquid crystal panels 17a, 17b, 17c, each of which is a transmission type display device; incident side polarizing plates 18a, 18b, 18c each disposed on the incident side of liquid crystal panel 17a, 17b, 17c; and exit side polarizing plates 19a, 19b, 19c each disposed on the exit side of liquid crystal panel 17a, 17b, 17c. Since a TN (Twisted Nematic) liquid crystal panel can exclusively handle a particular linearly polarized light component, respective color light fluxes from color separating optical system 7 are uniformly directed in a predetermined polarization direction (P-polarized light) by incident side polarizing plates 18a, 18b, 18c. After the P-polarized light is modulated by liquid crystal panels 17a, 17b, 17c, S-polarized light components of the modulated light is only allowed to pass through exit side polarizing plates 19a, 19b, 19c. 
While the optical configuration shown herein is associated with a three-plate type liquid crystal projector apparatus which separates light from a light source into three primary colors which are individually modulated by three liquid crystal panels, a light modulator in a similar configuration is also used in a low-luminance, inexpensive, single-plate type liquid crystal projector apparatus which employs only one liquid crystal panel.
In light modulator 14 in the configuration as described above, incident side polarizing plates 18a, 18b, 18c and exit side polarizing plates 19a, 19b, 19c tend to generate heat because each of these plates allows only polarized light in a single axial direction to pass therethrough and shields (absorbs) the remaining polarized light. Likewise, liquid crystal panels 17a, 17b, 17c involve the generation of heat when in operation, because transmitted light is shielded in a lattice-shaped wire called a black matrix which surrounds pixels on the panel. Liquid crystal panels 17a, 17b, 17c, incident side polarizing plates 18a, 18b, 18c, and exit side polarizing plates 19a, 19b, 19c are often made of organic materials, so that if these components are irradiated with short-wavelength light or exposed to a high-temperature environment for a long time, their functions are significantly compromised due to damaged panel alignment films, lowered polarized light selection characteristics, and the like. Such functional damages will result in a shorter lifetime of products, an increased running cost caused by frequent replacements of damaged components, a degradation in combined projection images due to variations in the respective color light characteristics. Therefore, some countermeasures must be taken against heat for these light modulators.
The following description will be made on a cooling method which has been conventionally employed for limiting a rise in temperature of the incident side polarizing plates, exit side polarizing plates, and liquid crystal panels (hereinafter these members are collectively called “liquid crystal unit 23”).
FIGS. 2A, 2B are explanatory diagrams generally illustrating an exemplary method of cooling a liquid crystal unit based on a forced air cooling scheme. FIG. 2A illustrates a perspective view of the cooling structure unit, wherein cooling fan 20, which comprises an axial fan or a radial fan, takes external air from a suction port of a liquid crystal projector housing (not shown), and introduces cooling air from an outlet port of cooling fan 20 to duct opening 22 beneath liquid crystal unit 23 through duct 21. As illustrated in FIG. 2B, incident side polarizing plates 18a, 18b, 18c, liquid crystal panels 17a, 17b, 17c, and exit side polarizing plates 19a, 19b, 19c, which make up liquid crystal unit 23, are spaced apart from one another such that the cooling air passes upward through gaps therebetween to take the heat on surfaces to be cooled, i.e., cool the members. An air flow, which has been heated by the heat exhausted from liquid crystal unit 23, is emitted to the outside of the housing from an exhaust port of the liquid crystal projector housing.
Other than the air cooling scheme, a liquid cooling scheme is also used for the liquid crystal unit. The liquid cooling scheme involves immersing heat-generating members of the liquid crystal unit in a coolant container filled with a coolant liquid to transport the heat generated in the liquid crystal unit through natural convection of the coolant liquid which has a high heat conductivity, and is disclosed, for example, in specification etc. of Japanese Patent Laid-open Publication No. 2002-214596. FIGS. 3A, 3B are conceptual diagrams illustrating an exemplary liquid cooling structure applied for cooling an exit side polarizing plate. In FIG. 3A, incident side polarizing plate 18d, liquid crystal panel 17d, exit side polarizing plate 19d, and color combiner prism 15 are spaced apart from one another in an order in which light travels therethrough. As illustrated in a partially enlarged view of FIG. 3B, exit side polarizing plate 19d is adhered to transparent substrate 30a of a cooling element, and cooled by the cooling element. The cooling element comprises metal frame 24a; two transparent substrates 30a, 30b adhered to openings of metal frame 24a with a gap therebetween; and coolant liquid 31 filled in an interrial space defined between transparent substrates 30a, 30b. 
FIG. 4 is a conceptual diagram illustrating another prior art liquid cooling structure which is disclosed in specification etc. of Japanese Patent Laid-open Publication No. 2002-357803. Liquid crystal panel 17e and exit side polarizing plate 19e are contained within metal frame 24b such that they are spaced apart from each other, and coolant liquid 31 is filled and enclosed in the internal space defined by liquid crystal panel 17e and exit side polarizing plate 19e to simultaneously cool liquid crystal panel 17e and exit side polarizing plate 19e. 
Either of the foregoing examples takes heat from members to be cooled through a coolant liquid, transports the heat to a metal frame by circulating the coolant liquid through convection, and cools the metal frame together with other liquid crystal unit members which are not immersed in the coolant liquid, by an air cooling fan disposed outside to dissipate the heat.
As disclosed in specification etc. of Japanese Patent Laid-open Publication No. 2003-195421, a method of improving cooling capabilities of a liquid crystal panel itself, which forms part of a liquid crystal unit, bonds a heat dissipating plate(s) or the like made of a transparent thin plate material having a thermal conductivity higher than liquid crystal panel components on one or both of the incident surface and exit surface of a liquid crystal panel. With the plate having a high thermal conductivity bonded to a heat generating surface, thermal diffusion on the panel surface is increased to improve the cooling capabilities.
Further, in a conventional liquid crystal projector apparatus, as shown in specification etc. of Japanese Patent Laid-open Publication No. 2003-75912, a liquid crystal panel, which forms part of a liquid crystal unit, is often provided with an adjusting function (adjustments of the optical axes) for adjustments in the vertical, horizontal, and rotating directions to permit alignments among respective pixels on the panel for a registration adjustment to precisely superimpose projected images from liquid crystal panels corresponding to the respective colors on a screen. FIGS. 5A, 5B illustrate an exemplary method of supporting a liquid crystal panel which can be optically adjusted. As illustrated in FIG. 5A, liquid crystal panel 17c is supported at a location spaced apart from color combiner prism 15. As illustrated in FIG. 5B which is a detailed cross-sectional view of the supporting structure, liquid crystal panel 17c is securely held by fixture 26c, and four corners of fixture 26c are supported at four points by holder protrusions 27, and are securely adhered after the optical axis has been adjusted. As a result, liquid crystal panel 17c itself is supported as if it were fixed in the air. To improve heat dissipation capabilities of this type of liquid crystal panel, a method has been disclosed for fixing the liquid crystal panel (fixture) and panel fixing frame (holder protrusions) with a thermally conductive adhesive resin.
In the prior art liquid crystal projector apparatuses described above, first of all, the following problems have been recognized in the liquid crystal projector apparatus which employs a forced air cooling scheme using a cooling fan.
A first problem is noise. A demand for higher luminance of liquid crystal projector apparatuses drives an increase in lamp power, whereas liquid crystal panels tend to be reduced in size in response to a demand for a reduction in the size of the apparatuses, necessarily resulting in an increase in the density of light flux incident on the liquid crystal unit which suffers an increased heat load. In the forced convection using a cooling fan, since its average thermal conductivity is proportional to ½ power of wind speed, the wind speed must be increased for effectively dissipating heat generated in the liquid crystal unit. While there are two methods contemplated for increasing a cooling wind speed, i.e., the employment of a larger fan, and an increase in the wind speed, the former method is not compatible with the demand for a reduction in size, and may often encounter difficulties in physically installing a larger fan in a housing. Therefore, a small fan is rotated at a higher speed to increase the amount of supply air to ensure the cooling capabilities. However, an increase in the rotational speed of the fan causes increased noise which exacerbates the comfort of the user.
A second problem is the reliability. When a light modulator is cooled down through forced air cooling in a liquid crystal projector apparatus, a problem is caused by dust particles which are mixed in cooling air taken from external air. A liquid crystal light valve has a pixel size of 26 μm square in a 1.3″ XGA, so that if dust particles of a size on the order of several tens of micrometers stick on an imaging surface of the panel, enlarged shadows of the dust particles are projected onto a screen together with an image to degrade the image quality. To solve this problem, a finer filter may be attached to a suction port for enhancing a filtering function, but will cause another problem of an increased loss of suction pressure of the fan to cause a reduced amount of supply air and a resulting failure in providing sufficient cooling performance.
On the other hand, a liquid crystal projector apparatus which employs a liquid cooling scheme implies the following problems. A first problem is the image quality. When a coolant liquid is filled between components of a liquid crystal unit (for example, between liquid crystal panel 17e and exit side polarizing plate 19e in FIG. 4), a liquid layer intervenes in a light transmission space, and therefore causes disturbed polarization of light which transmits the liquid layer due to variations in coolant density arising from generation of air bubble and thermal transportation, and due to non-uniform convection resulting therefrom, resulting in discrepancy in image information which passes through the exit side polarizing plate, and a degraded quality of a projected image.
A second problem is the reliability and mountability. Since the heat is dissipated from the liquid crystal unit through the encapsulated coolant liquid, the coolant liquid repeats expansion and contraction as it is used. While a coolant container is provided with a pressure adjusting mechanism for accommodating the expansion and contraction of the coolant liquid, a pressure adjusting member may be damaged in a long-term service to possibly cause a leak of the coolant liquid. In addition, a complicated sealing structure and pressure adjusting structure of the coolant container could significantly impair the ease of assembly of an optical engine.
Further, the aforementioned liquid crystal projector apparatus of the type which fixes a liquid crystal panel by a holder member in the air has problems in that the connection areas of the protrusions which support the liquid crystal panel at four points are not sufficient to absorb the heat generated in the liquid crystal panel, and that a sufficient thermal connection cannot be ensured due to limitations imposed by the heat conducting performance of the adhesive resin. In addition, when the liquid crystal panel is fixed by the holding mechanism of the holder member in a manner similar to the other polarizing plates, the liquid crystal panel can be affected by the rigidness of the holding mechanism, resulting in the possibility in going out of alignment during assembling or in operation. However, it is difficult to accomplish a thermal connection without affecting the posture of the liquid crystal panel fixed in the air.